Three dimensional optical imaging systems, hereinafter referred to as “3D cameras”, that are capable of providing distance measurements to objects and points on objects that they image, are used for many different applications. Among these applications are profile inspection of manufactured goods, CAD verification, robot vision, geographic surveying and imaging objects selectively as a function of distance.
Some 3D cameras use a time of flight technique to measure distances to an object in a scene. A 3D camera of this type usually comprises a light source, such as a laser, which is pulsed or shuttered to illuminate a scene being imaged with light pulses and a gated imaging system for imaging light from the light pulses that is reflected from objects in the scene. The gated imaging system comprises a camera, such as a CCD or CMOS camera, or an IR camera, having a photosensitive surface, hereinafter referred to as a “photosurface”, and a gating means for gating the camera open and closed, such as an electro-optical shutter or a gated image intensifier. The reflected light is registered on pixels of the photosurface of the camera only if it reaches the camera when the camera is gated open.
To image a scene and determine distances from the camera to objects in the scene, the scene is generally illuminated with a train of light pulses radiated from the light source. For each radiated light pulse in the train, following an accurately determined delay from the time that the light pulse is radiated, the camera is gated open for a limited period of time hereinafter referred to as a “gate”. Light from the light pulse that is reflected from an object in the scene is imaged on the photosurface of the camera if it reaches the camera during the gate. Since the time elapsed between radiating a light pulse and the gate that follows it is known, the time it took imaged light to travel from the light source to the reflecting object in the scene and back to the camera is known. The time elapsed is used to determine the distance to the object.
Some 3D cameras use a geometrical relationship between an incident beam of light that illuminates the surface of an object and light reflected from the incident beam by the surface to determine distances to the object. Generally, at any one time, the incident beam illuminates a limited surface region of the object and distances to the object are measured only for points of the object in the limited surface region. To measure distance to points in different surface regions the position of the beam is changed. Generally, “geometrical” measurements of distances to an object are made using a triangulation technique.
A common “geometrical” method of 3D imaging is active triangulation. In active triangulation, generally, a thin fan beam of laser light illuminates the surface of an object along a thin stripe on the surface. Light reflected from the illuminated stripe is incident on pixels in an appropriate photosurface. Each illuminated pixel in the photosurface is illuminated by light reflected from a different spot on the stripe. The position of an illuminated pixel in the photosurface and the angle, hereinafter the “scan angle”, that the plane of the fan beam makes with a reference coordinate system is sufficient to determine three spatial coordinates for the spot on the stripe that reflects light to the pixel. To produce a 3D map of the surface of the object, the scan angle is incremented so that the fan beam scans the surface of the object, illuminating it successively along different closely spaced stripes on the surface. For each of these closely spaced stripes, the spatial coordinates of spots on the stripes corresponding to illuminated pixels are calculated.
3D cameras using time of flight and variations of time of flight techniques to determine distances to an object are described in PCT application PCL/IL98/00476 by some of the inventors of the present application, which PCT application is incorporated herein by reference.
3D cameras using substantially “geometrical” methods for measuring distances to an object are described in PCT application PCT/IL97/00370, also by some of the inventors of the present application, which PCT application is incorporated herein by reference.